9410
J . Am. Chem. SOC.1990, I 12, 94 10-94 1 1
Foundation (CH-8613468) to D.H.W. We thank the CNR (Italy) for a postdoctoral fellowship to P.T., Professor Stephen Weber (Pittsburgh) for many helpful discussions, and Simon Hirst for his assistance.
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IA
Long-Range Electronic Interactions in Peptides: The Remote Heavy Atom Effect G a u t a m Basu, M a t t Kubasik, Demetrios Anglos, Beth Secor, and Atsuo Kuki*
Cornell University, Department of Chemistry Baker Laboratory, Ithaca, New York 14853 Received February 16, 1990 Introduction. A proper understanding of electronic interactions between nonconjugated molecular components depends heavily upon the ability t o design and build molecular structures that are rigid and whose intercomponent geometry is well defined. The intramolecular heavy atom effect,' a variant of t h e extensively studied intermolecular heavy atom effect (S, T I ) , * enables a direct study of the dependence of remote electronic interactions upon molecular structure, but has only been studied at a separation of three u bonds or less. Polypeptides, a basic architectural unit in nature, have been exploited in the past to study singlet excitation transfer,j electron transfer: and excitonic interactions.s We chose to focus on rigid helical oligopeptides which fold to bring two electronic partners positioned one turn a p a r t into close proximity even though the through-bond separation may remain large. This offers t h e opportunity to explore mechanisms of electronic interactions such as the question6 of covalent (through-bond) vs noncovalent (through-space) mechanisms. A further motivation is that the long-range spin-exchange interactions responsible for the remote heavy atom effect may be intimately connected to and shed light upon mechanisms of long-range electronic tunneling.' We report here bromine-induced enhanced intersystem crossing in the naphthalene chromophore at 13 u bond separation in aaminoisobutyric acid (Aib) rich oligopeptides containing @-(1'naphthyl)-L-alanine ( N a p ) and p-bromo-L-phenylalanine (Bph). The Ground State. I n the present study, two octapeptides and two dipeptides were synthesized.8 The dipeptides, H-Nap-BphOMe (bromo dimer, Br-Dim) and H-Nap-Phe-OMe (control dimer, C-Dim), contained no Aib residues and thus had no conformational bias. The control and the bromo octamers ( C - O c t and Br-Oct, respectively) contained six host Aib residues to induce
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( I ) (a) Kavarnos, G.;Cole, T.; Scribe, P.; Dalton, J. C.; Turro, N . J . J . Am. Chem. Soc. 1971, 93, 1032. (b) Davidson, R. S.; Bonneau, R.; Joussot-Dubien, J.; Trethewey, K. R. Chem. Phys. Lett. 1980, 74, 318. (2) (a) McGlynn, S.P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy o/the Triplet Slate; Prentice Hall: Englewood Cliffs, N J , 1969. (b) Martinho, J. M.G. J . Phys. Chem. 1989, 93, 6687 and references therein. (c) For the bimolccular system of bromobutane and a-methylnaphthalene in acetonitrile at room temperature, we have measured the external heavy atom induced intersystem crossing rate to be 1.1 X IOs M-I s-l. (3) Stryer. L.; Haugland, R. P. Proc. Natl. Acad. Sci. U.S.A. 1967, 58, 719. Sisido, M.; Yagyo, K.; Kawakubo, H.; Imanishi, Y. Inr. J . Biol. Mucromol. 1982, 4, 3 13. (4) Sisido, M.; Tanaka. R.; Inai, Y . :Imanishi, Y. J . Am. Chem. Soc. 1989, 111, 6790. Isied, S.S.;Vassilian, A. /bid. 1984, 106, 1732. Faraggi. M.; DeFelippis, M. R.; Klapper, M. H. /bid. 1989, 111, 5141. Schanze, K. S.; Sauer, K. /bid. 1988, 110, 1180. (5) Sisido, M.; Egusa, S.;Imanishi, Y . J . Am. Chem. Soc. 1983.105, 1041. (6) Kuki, A.; Wolynes, P. G. Science 1987, 236, 1647. Closs, G . L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986. 90, 3673. Beratan, 0. N.; Onuchic, J. N.;Hopfield, J. J. J . Chem. Phys. 1987,86, 4480. (7) This correlation is expected to be true to the extent that long-range spin exchange depends. from the perspective of second or higher order prturbation theory, on Hamiltonian matrix elements to virtual electron transfer states. Haberkorn, R.; Michel-Beyerle, M. E.; Marcus, R. A. Proc. Narl. Acad. Sei. U.S.A. 1979, 76.4185. Clou, G.; Piotrowiak, P.; MacInnis, J. M.; Fleming. G.R. J . Am. Chem. Soc. 1988, 110, 2562. (8) Solution-phase peptide synthesis techniques" were employed. Aib dimers were made by azirine" or oxazoloneIk chemistry, and three such dimer blocks were coupled with the two appropriate guest amino acids by oxazolone or acid chloridei6 methods to produce the target octamers. Full synthetic details will be presented elsewhere.
0002-78631901 I 5 1 2-94 I0$02.50/0
Ac-Aib-Aib-Nap-Aib-Aib-Phe-Aib-Aib-NHMe
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Figure 1. The Aib-rich control octamer (C-Oct) displayed in a 3,0 helical conformation ( i i 3 hydrogen-bonding pattern). This structure is consistent with 'H NMR data and exploratory energy minimization results'7 and is similar to that determined by Toniolo and co-workers for thep-BrBz-(Aib),-O-f-Bu octamer.I2 The pitch of the 310 helix is close to 6 8, (5.85 8, in the X-ray structure of Toniolo's octamer). The Br represents the location of the bromine in Br-Oct, in which Bph replaces Phe. The dark atoms are carbonyl oxygens. Table I. Steady-State and Time-Resolved Fluorescence Data"qb solvent CH3OH CH3CN CH,CN/THF ( 1 : l ) THF THF/isooctane ( 1 : l )
relative fluorescence Quantum vieldsC Br-Oct/C-Oct Br-Dim/C-Dim" 0.36 0.32 0.34 0.28 0.28
0.79 0.82
lifetime: iF. ns solvent Br-Octc( C-Oct CH3OH 28.4 f 0.6 60.6 f 0.6 CHICN 26.6 f 0.6 54.4 f 2.7 "All samples were degassed by freeze-pump-thaw. A,, = 290 nm. bThe concentrations (-10 pM) were far below that required to initiate self-aggregation (0.6 mM) in similar octapeptides.lDa The fluorescence quenching in the octamer was also measured to be concentration independent from 5 pM to 20 pM.
